poor mileage

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  #1  
Old 06-07-09, 06:14 PM
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poor mileage

03 Bonneville 3.8L 100,000 miles. I have question on my car. On 2 hour trips to St.louis my car used to get 29 mpg easy. Then Missouri passed a mandate on ethanol in fuel. My car went down to 25 mpg. On a trip out of state it went back to 29 mpg. In the last few months it has went to 23 mpg. I noticed the catalytic converter rattling inside it so I had it replaced along with the fuel filter. Still no better on the mileage. Does anyone have any ideas? No alerts are coming from the computer system check. I was thinking of getting the injectors cleaned. Thanks for any input.
 
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Old 06-07-09, 06:57 PM
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Has the O2 sensor ever been replaced?
 
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Old 06-07-09, 07:27 PM
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No it has not. Can that be bad and not show up on anything?
 
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Old 06-07-09, 08:20 PM
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Ethanol fuels get fewer mpgs than non ethanol fuels, this is because gallon per gallon there is less energy in ethanol as compared to 100% gasoline. Another thought, have you changed the sparkplugs? Most cars now have a 100000 mile tune up interval, might help.
 
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Old 06-07-09, 09:31 PM
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Originally Posted by wire twister View Post
Ethanol fuels get fewer mpgs than non ethanol fuels, this is because gallon per gallon there is less energy in ethanol as compared to 100% gasoline. Another thought, have you changed the sparkplugs? Most cars now have a 100000 mile tune up interval, might help.

On the money.....

Dont expect comparable mileage from ethanol.....It just aint gonna Happen......
 
  #6  
Old 06-08-09, 07:34 PM
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I'm not sure if a bad O2 sensor will always show up on OBD II. From what I've read, they can get 'slow' but not completely dead so maybe... They typically are replaced before 100K miles and are often the culprit when fuel mileage degrades. The owners manual probably says something about when they should be replaced.
 
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Old 06-09-09, 05:42 PM
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Thanks for you advice I will check into it. It used to get 25 mpg even with the 10% ethanol but not anymore. I read where my state is considering making the ehanol mandate 15% now. Previously my state secretly put ethanol in the fuel before the 10% mandate to get you used to it. I wonder if that is whats going on this time.
 
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